Prefer tag/size outside of union where possible

If we have control of the struct, such as in lfsr_data_t and lfsr_cat_t,
moving the common tag outside of the union avoids naming ambiguities.

Counter-example: This doesn't work for lfsr_bshrub_t, since the contents
of that union are also used as separate types elsewhere. Fortunately the
common initial sequence union rules kick in here.

No code changes, which is good:

           code          stack
  before: 33652           2624
  after:  33652 (+0.0%)   2624 (+0.0%)

lfs.h

@@ -397,17 +397,13 @@ typedef struct lfs_mdir {
 typedef struct lfsr_data {
     // sign(size)=0 => in-RAM buffer
     // sign(size)=1 => on-disk reference
+    lfs_size_t size;
     union {
-        lfs_size_t size;
         struct {
-            lfs_size_t size;
             lfs_block_t block;
             lfs_size_t off;
         } disk;
-        struct {
-            lfs_size_t size;
-            const uint8_t *buffer;
-        } buf;
+        const uint8_t *buffer;
     } u;
 } lfsr_data_t;